It happens with a 13 speed of you're downshifting while jaking to a gradual stop, you go to drop half a gear but the throttle doesn't respond as quickly as usual and you don't quite reach the higher RPM required for the splitter to "grab". I don't really know how to explain it better. But then the engine falls to idle and you can hear that something ain't right in the trans.
The only way to correct this when it happens is to step on the throttle and get it to reach the proper RPM so it grabs, or push the lever into neutral and flip it back forward, then match RPM with your foot and re-engage.
Never hit the splitter back forward while it's idling. It suddenly engages and instantly jerks the engine back up from idle.
I did that once and I was surprised it didn't destroy the truck.